In order to implement a 2-wire video intercom system, the direct current power, the video carrier signal, the audio signal and the command data signal have to be transmitted on a common 2-wire bus simultaneously. Therefore, the power supply circuits of the system power supply and devices thereof have to comprise an inductance component connected to the common bus in series, which allows the direct current passing whilst also suppresses the alternating current signal. However, it is common sense to the skilled person in art that the audio signal frequency is low to 300 Hz if a coil inductor is used as the inductance component. In order to achieve the sufficient impedance and power supply capacity as well, the size of such inductor shall become very large. Usually an electronic inductance circuit can be used to replace the coil inductor to reduce its size. Due to the conventional inductance circuit with an alternating current feedback resistor in series in the main current path, the direct current power supply capability and the alternating current impedance are limited; hence the size of 2-wire video intercom system is limited.
In particular, U.S. Pat. No. 6,087,823A published a conventional electronic inductance circuit. FIG. 1 and FIG. 2 illustrate a kind of conventional electronic inductance circuit individually. As shown in FIG. 1, the electronic inductance circuit EL1 comprises a P-channel FET Q1, resistors R1, R2 and a capacitor C1, wherein the terminal AI as an input terminal and the terminal AO as an output terminal. Between terminals AI and AO, a main current path is formed along the drain terminal D and the source terminal S of the P-channel FET Q1 and the resistor R1 connected in series. Also between terminals AI and AO, the resistor R2 and the capacitor C1 are connected in series, which is connected to the main path in parallel. Furthermore, the connection node B1 between the resistor R2 and capacitor C1 is connected to the Gate terminal G of the FET Q1.
Taking FIG. 1 as an example, when connecting AI to a regulated direct current power supply and connecting AO to a device load, the voltage across the capacitor C1 can't be changed transiently; i.e., UC1=0, UGS=0 and Q1 is still turned off. The voltage of AO will be dropped to the reference GND by device load that UAI−UAO=UAI=UR2, so C1 will be charged by the resistor R2. When the voltage across C1 is larger than the gate threshold voltage of the FET Q1, the Q1 starts to be turned on. When the current I1 reaches the required current value of the device load, the charging of the capacitor C1 will be stopped and UGD=0, therefore, the voltage drop of the electronic inductance circuit EL1 is represented by equation (1) as below:UEL1=UAI−UAO=URI+USG=I1*R1+USG  (1)
Next, as to the alternating current impedance of the electronic inductance EL1, if a fluctuation voltage ΔU occurs at the terminal of AO, then the fluctuation voltage across C1 is ΔUC1=ΔU*ZC1/(R2+ZC1). Meanwhile ΔUC1=ΔUSG+ΔUR1=ΔI1/gm+ΔI1*R1, so ΔU*ZC1/(R2+ZC1)=ΔI1/gm+ΔI1*R1. Therefore, the alternating current impedance ZEL1 between the terminals AI and AO is represented by equation (2) as below:ZEL1=(R1+ZQ1)//(R2+ZC1)={(1+R1*gm)/gm}*{(R2+ZC1)/ZC1}//(R2+ZC1)  (2)
Wherein, ZC1=1/(j*ω*C1)=1/(j*2*π*f), “gm” represents the trans-conductance of the FET Q1.
When the electronic inductance circuit EL1 allows direct current passing, it is preferable to make the voltage drop UEL1 represented by equation (1) small and direct current respond promptly. On the other hand, the alternating current impedance ZEL1 represented by equation (2) shall be sufficiently larger than the cable loop resistance of intercom system, what's more it does not change with the direct current changing.
FIG. 2 illustrates a similar electronic inductance circuit EL2 to the one in FIG. 1. The main differences lie in that an N-channel FET in the electronic inductance circuit EL1 is used instead of a P-channel FET Q1 in FIG. 1. Correspondingly, each part of the circuit in FIG. 2 is arranged contrarily to the one in FIG. 1. In this way, the equations (1) and (2) described above are also applicably to represent the voltage drop and alternating current impedance between the terminal AI and AO of the electronic inductance circuit respectively.
According to the above description, it's obvious to the skilled person in art that the voltage drop UEL1 between the terminals AI and AO of the electronic inductance circuit EL1 represented by the above equation (1) is the sum of voltage drop across R1 and USG during the current is I1. Usually, we can select a FET with an appropriate UGS so that the voltage drop USD between the FET source terminal and drain terminal is also appropriate, hence the audio signal transmitted at the bus will not be distorted and the voltage is not too large. But the voltage drop across R1 is linearly proportional to vale of R1 and I1, when a large scale of intercom system is wanted with dozens of video intercom devices in parallel connected to the common bus, the direct current I1 will become very large; so does the voltage drop of R1, which means the consumption of R1 will become large.
In order to decrease the voltage drop and consumption of R1, the R1 has to be very small. But according to the equation (2), if the R1 is not sufficiently large, the ZEL1 is also proportional to the value of R1. If the R1 is reduced insufficiently, the ZEL1 will be insufficient. According to the equation (2), if R1 is not sufficiently large, the ZEL1 will be changed and depend on the trans-conductance gm of FET Q1. Because the gm is affected by the direct current IL through the Q1, so the ZEL1 will decrease with the increase of the direct current IL. If the direct current larger than 1 A is desired, the ZEL1 will be insufficient for the audio signal transmission.
Furthermore, the ZEL1 has to be sufficient for the audio signal transmitted in a low frequency as 300 Hz, but according to the equation (2), the ZEL1 is a first-order relationship with frequency. Hence, the ZEL1 decreases from 300 Hz to a lower frequency slowly, this means the electronic inductance circuit response to direct current power supply is very slow.
In summary, according to the equation (2), the resistance of R1 has to be sufficiently large to achieve a sufficient alternating current impedance in the conventional electronic inductance circuit with a resistance component for alternating current feedback, whilst according to the equation (1), the resistance of R1 has to be sufficiently small to achieve a sufficiently low voltage drop and power consumption. Consequently, existing solutions including the prior art mentioned above can't supply large direct current with sufficient alternating current impedance simultaneously. Due to the above mentioned problems, the present invention is to propose an electronic inductance circuit for the power supply of a 2-wire bus intercom system and a device thereof.